The present invention relates generally to earth moving equipment and more particulary pertains to a system for automatically maintaining an earth mover's blade height constant independent of the earth mover's change in pitch angle.
Earth moving equipment is often called upon to perform ground conditioning operations such as the preparation of accurately finished substantially flat ground surfaces. The accuracy required for the production of building lots or for road grading requires a high degree of skill in the operation of the earth moving equipment. It is especially difficult to manually keep the blade of a bulldozer at a controlled, substantially fixed level as the bulldozer body is subjected to pitching when traversing a rugged ground surface. Underskilled operation of a bulldozer over a rugged terrain can actually cause the surface to become even more uneven as the bulldozer rides up over high spots and down through low spots with commensurate changes in blade height and inclination. In order that a rugged ground surface may be flatly bulldozed, an operator is required to operate an operating lever of the bulldozer to control the earth removing blade thereof while the operator carefully investigates the state of that portion of the ground surface which is located forwardly of the earth removing blade and which is included in the dead angle of sight as viewed from a normal operating position and while the operator watches the movement of the earth removing blade relative to the rugged ground surface. Problems are compounded when grading a surface to a predetermined slope angle. It takes a great amount of time for a man to become skilled in such a complicated control operation. The blade control operation is particularly difficult and requires highly skilled technique when the bulldozer moves at a higher speed.
A variety of systems have been disclosed that automatically control the operation of the blade. Relatively elaborate devices utilizing inertial systems or remote points of reference enable a substantially flat or level surface to be produced with a minimum of operator intervention. On the other hand, it has been found that devices which serve to merely stabilize the blade relative to the horizontal provide sufficient control to yield satisfactory results. Such blade stabilizers thereby provide a relatively inexpensive alternative to systems employing laser guidance or inertial techniques.
Typically, the stabilizing systems employ an inclinometer in one form or another to measure the angle of the blade relative to the horizontal. Output from the inclinometer is interpreted accordingly and ultimately employed to control the hydraulics that operate the blade. The conditions under which these devices are called to function are harsh. While in operation, a piece of earth moving equipment experiences extremes in vibration, shockloads, temperature and contamination. To provide satisfactory performance, the control devices must withstand the same adverse conditions while providing continuous and reliable control.
The device described in U.S. Pat. No. 4,524,836 relies on a tilt vial to supply information regarding a blade's angular deviation from a predetermined value. Such a tilt vial typically takes the form of a mercury switch in which a closed capsule is substantially but not completely filled with the electrically conductive liquid. Any deviation from the horizontal causes the liquid to flow to one end of the capsule where it serves to close one of the two diametrically opposed electric circuits. The disadvantage inherent in such a sensor is that vibration and shock loading causes the liquid level to fluctuate wildly within the capsule thereby causing both electric circuits to open and close intermittently. Furthermore the signals produced by such a device are limited to a binary output which provides no information regarding the magnitude of the deviation. Consequently, the hydraulic actuators can only be controlled in a rather coarse fashion which may afford erratic operation.
U.S. Pat. No. 4,535,847 discloses a inclinometer employing a float and proximity switches. This design suffers from similar shortcomings in that it is adversely affected by shock and vibration and is capable of providing only binary information.
The device described in U.S. Pat. No. 4,024,823 employs a manometer in concert With a light source and photocells. The manometer is positioned such that the level of an opaque fluid therein varies as a function of the blade height. The level of the opaque fluid rises and falls between the light source and the photocells, consequently the higher the fluid level the lower the photoelectric output. While this does provide linear information regarding the magnitude of angular deviation from the horizontal, it is a rather complex and delicate apparatus that seems ill-suited for use on an earth mover. U.S. Pat. No. 3,831,683 employs gyroscopes which add an undesirable amount of complexity to such a device. An increased degree of complexity augments many of the above-indicated typical shortcomings of the prior art. Besides increasing the cost of manufacture, more parts and complex interactions thereof increase their susceptibility to failure due to vibration, shock, temperature and contamination.
The size and bulk of some of these prior art devices poses further problems. As it is usually necessary for such devices to be positioned on or about the earth mover blade's actuation system and it is desirable that the device be located substantially out of harm's way, it is most advantageous that its overall dimensions be minimized. Difficulties associated with size and bulk are compounded for devices intended for retrofit applications where no special niches had been provided on the earth mover.